Abstract
The lightest elements that make up our universe were created in the Big Bang. Elements heavier than Li-7 are produced every day in the heart of stars. Elements are synthesised in different nuclear processes taking place in different life stages of stars. Among other particles produced during these reactions, neutrons are readily available in stars and their velocities follow a Maxwell-Boltzmann distribution, centered around the value that corresponds to the star's temperature. One of the most important nucleosynthesis processes is the s-process, consisting of neutron captures and subsequent decays. Astrophysical models can deduce the elemental ratios and model the chemical evolution of our universe, but in order to accurately do so, they need input from nuclear reaction studies, such as accurate cross section values for astrophysical processes. Since neutrons in stars follow a Maxwellian distribution, these cross sections are referred to as Maxwellian-averaged cross sections, or MACSs. ...
The lightest elements that make up our universe were created in the Big Bang. Elements heavier than Li-7 are produced every day in the heart of stars. Elements are synthesised in different nuclear processes taking place in different life stages of stars. Among other particles produced during these reactions, neutrons are readily available in stars and their velocities follow a Maxwell-Boltzmann distribution, centered around the value that corresponds to the star's temperature. One of the most important nucleosynthesis processes is the s-process, consisting of neutron captures and subsequent decays. Astrophysical models can deduce the elemental ratios and model the chemical evolution of our universe, but in order to accurately do so, they need input from nuclear reaction studies, such as accurate cross section values for astrophysical processes. Since neutrons in stars follow a Maxwellian distribution, these cross sections are referred to as Maxwellian-averaged cross sections, or MACSs. MACSs can be calculated by folding point-wise cross section data with a Maxwellian distribution or can be directly measured, if a Maxwellian neutron beam is available. The neutron time-of-flight facility (n_TOF) is CERN's neutron source. Based on a proton beam from the proton synchrotron (PS) impinging on a lead spallation target, n_TOF comprises three experimental areas, two at the end of long flight paths in order to perform measurements using the time-of-flight (TOF) technique, and one right next to the spallation target itself, benefiting from high neutron flux. This third experimental area, the "NEAR" station, could potentially be used to perform integral MACS measurements on cases that are too challenging to measure via TOF. A prerequisite to this is to filter the neutron energy distribution into a Maxwellian one. This work is a feasibility study of such a filtering technique. In order to shape the neutron energy distribution, filters made of B4C, enriched in B-10 and thus highly interacting with low energy neutrons, were used. To test the shape results, neutron capture reactions of already known point-wise cross sections and MACS were measured. The methodology for these measurements was the activation technique, which consists of two steps: Irradiating the sample and afterwards measuring its induced activity. We can then deduce the number of nuclei of interest that was produced during the irradiation. This number is related to the cross section of the reaction for this specific shape of neutron flux. By comparing the experimental results with the ideal-case results we would have gotten if we had had a perfect Maxwellian beam, we can quantify the quality of the shaping technique used in this study and make inferences as well as future suggestions. The conclusions of this work can be summarised in the following two points:- MACS can be measured at n_TOF only within a factor 2-3. This is not a high accuracy measurement, it can however be useful in cases where the MACS is completely unknown, or deduced with a large uncertainty.- Thicker filters lead to smaller differences between experimental and ideal conditions. It should be noted though that increasing the thickness of the filters too much can have the opposite effect, as they then start to interfere too much and lead to further scattering or background issues. - The experimental conditions can be significantly improved by the installation of a moderating system that will further shape the beam by interacting with the higher-energy neutrons, in addition to the filter that can absorb low energy ones.
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